A self-supporting electrically-conductive network reinforced gas diffusion electrode for electrochemical conversion pro- cesses

The present invention relates to gas diffusion electrodes for electrochemical conversion processes. In modern times, electrochemical conversion processes are used for various purposes, for example electrochemical con ^¬ version of Carbon dioxide to Carbon monoxide in an electro ^¬ chemical conversion device. The electrochemical conversion devices in which such electrochemical conversion processes are performed, for example fuel cells, include special elec ^¬ trodes, i.e. gas diffusion electrodes, with a conjunction of a solid, liquid and gaseous interface, and include an elec ^¬ trical conducting catalyst that supports an electrochemical reaction between the liquid and the gaseous phases or between the gaseous and the gaseous phases for the electrochemical conversion .

Various types of gas diffusion electrodes are used in differ ^¬ ent electrochemical conversion processes and various tech- niques are used to fabricate such gas diffusion electrodes. In one technique, a first layer having electrical conductive property is formed on a pre-formed conductive solid support. The first layer is formed by casting the first layer on the pre-formed conductive solid support by using a casting sus- pension that has electrically conductive particles dissolved in an organic binder material solution. Alternatively, the first layer may be formed by setting the pre-formed conduc ^¬ tive solid support in the casting suspension. The first layer is formed on and around the pre-formed conductive solid sup- port. Pores are then realized in the first layer by removing the solvent of the organic binder material of the first lay ^¬ er. Independent of the formation of the first layer, a second layer having hydrophobic property is formed. The second layer is formed by casting the second layer on an inert surface such as glass surface. For casting the second layer, a cast ^¬ ing suspension that has hydrophobic particles in a solution of an organic binder material is used. Pores are then real- ized in the second layer by removing the solvent of the or ^¬ ganic binder material of the second layer. Thereafter, the first layer along with the conductive solid support and se ^¬ cond layer are pressed together to form the gas diffusion electrode .

In an alternate technique, a first layer having electrical conductive property is formed on a pre-formed conductive sol ^¬ id support. The first layer is formed by casting the first layer on the pre-formed conductive solid support by using a casting suspension that has electrically conductive particles suspended in an organic binder material solution. Alterna ^¬ tively, the first layer may be formed by setting the pre ^¬ formed conductive solid support in the casting suspension. The first layer is formed on and around the pre-formed con- ductive solid support. Thereafter on top of the first layer, a second layer having hydrophobic property is formed. The se ^¬ cond layer is formed by casting the second layer on top of the first layer by using a casting suspension that has hydrophobic particles in a solution of an organic binder material. Thereafter, the solvents of the organic polymer binding mate ^¬ rials are removed from the first and second layers to realize porosity in the layers and finally forming the gas diffusion electrode supported by the pre-formed conductive solid sup ^¬ port .

The pre-formed conductive solid support, usually having a mesh-structure or web structure and generally formed of per ^¬ forated metallic sheets or meshes, provides mechanical strength and physical integrity to the gas diffusion elec- trode, hereinafter also referred to as the GDE, besides providing electrical conductivity to parts of the GDE for ex ^¬ ample the pre-formed conductive solid support may also func ^¬ tion as a current collector for the fuel cell. Such GDEs with conductive solid support are mechanically stronger but suffer from other disadvantages. One such disadvantage is that di ^¬ mensions of such GDEs are limited by dimensions of the pre ^¬ formed conductive solid support. Furthermore, setting of the pre-formed conductive solid support in the casting suspension is complicated and needs to be carried out with utmost pre ^¬ cession to obtain a uniformly formed first layer and thus a uniformly formed GDE . Often the thickness of the first layer on opposing sides of the pre-formed conductive solid support is different and thus results in a non-uniform GDE which is not optimally efficient for usage in the electrochemical con ^¬ version processes. Also, the pre-formed conductive solid sup ^¬ port adds to the cost of such GDEs. Furthermore, the electrically conductive particles in the first layer besides conducting electricity may also have cat ^¬ alytic property for the electrochemical reaction of the elec ^¬ trochemical conversion process; alternatively separate parti ^¬ cles of catalytic material may be added to the first layer besides the electrically conductive particles. Therefore, the presently known GDEs employ or need multiple different ele ^¬ ments or constituents of the presently known GDEs to provide physical integrity, mechanical strength, electrical conduc ^¬ tivity, and catalytic property to the GDE.

Thus there is a need for a gas diffusion electrode for use in electrochemical conversion processes that is physically strong and does not suffer from limitations contributed by the above mentioned pre-formed conductive solid support. It is also desirable that the need for having multiple different elements or constituents, as in case of the presently known GDEs, to provide physical integrity, mechanical strength, electrical conductivity, and catalytic property to the GDEs is obviated.

Thus the object of the present disclosure is to provide a GDE and a method for making the GDE for use in electrochemical conversion processes. The GDE of the present technique is de- sired to obviate the need of having multiple elements for providing physical integrity, mechanical strength, electrical conductivity, and catalytic property to the GDE. The above object is achieved by a method for making a gas diffusion electrode for use in an electrochemical conversion process according to claim 1 and a gas diffusion electrode for use in an electrochemical conversion process according to claim 15 of the present technique. Advantageous embodiments of the present technique are provided in dependent claims.

In an aspect of the present technique, a method for making a gas diffusion electrode for an electrochemical conversion processes is presented. In the method, interconnecting enti- ties, for example fibers or flakes, and an amount of the in ^¬ terconnecting entities are selected. The interconnecting entities, hereinafter also referred to as the IEs are selected such that the IEs include a material having catalytic proper ^¬ ty for the electrochemical conversion process. Thereafter, a casting suspension is prepared by mixing the IEs into a solu ^¬ tion. The solution includes an organic polymer binding material as a solute and a solvent for the organic polymer bind ^¬ ing material. Then, a sheet having predetermined dimensions is formed by spreading the casting suspension on an inert surface. The IEs and the amount of the IEs added to form the casting suspension are selected such that the IEs physically contact each other to form a self-supporting electrically- conductive network of the IEs when the casting suspension is spread in the predetermined dimensions to form the sheet. Fi- nally in the method, the solvent from the sheet is extracted by removing the solvent form the sheet to form the GDE.

The IEs, owing to their structure and the amount selected, form the self-supporting electrically-conductive network that provides mechanical strength and physical integrity to the GDE, and thus the requirement of having a pre-formed conduc ^¬ tive solid support is obviated and furthermore the dimensions of the GDE of the present technique are not limited by the dimensions of the pre-formed conductive solid support. Fur ^¬ thermore, the self-supporting electrically-conductive network formed within the sheet, because of being formed along with the casting of the sheet, is present within or spread

throughout or covers from within almost the entire volume of the sheet and thus provides better physical integrity to the organic binder material layer with pores as compared to a pre-formed conductive solid support that only provides a base support, i.e. acts as a base for the first layer, and does not run through the entire volume of the first layer. Also, the need for separately having electrically conductive parti ^¬ cles, besides the pre-formed conductive solid support, to conduct electricity through the GDE is also obviated because of the presence of the self-supporting electrically- conductive network formed within the volume of the sheet.

Moreover, since the IEs are formed of material having cata ^¬ lytic property for the electrochemical conversion process, the need for having separate catalytic particles is also ob ^¬ viated. Thus, the self-supporting electrically-conductive network formed within the sheet of the GDE of the present technique provides physical integrity, mechanical strength, electrical conductivity, and catalytic property to the GDE and thereby obviating or minimizing the need of having multiple elements for these purposes as is in case of the conven- tionally known GDEs.

In an embodiment of the method, the IEs are fibers, or at least some of the IEs are fibers. The fibers may have an un- branched or branched structure. A net length of the fibers, i.e. simply length of the un-branched fiber or total length of all the branches added with length of the part of the branched fiber from which the branches arise, is between 0.1 micrometer and 1000 micrometer. The average diameter of the fibers, i.e. simply diameter of the un-branched fiber or av- erage of diameters of all the branches and of diameter of the part of the branched fiber from which the branches arise, is between 0.05 micrometer and 50 micrometer. The un-branched fibers are easy to fabricate, whereas the branched structured fibers provides greater interconnectivity between the differ ^¬ ent IE units for example between a first branched fiber and a second branched fiber. In another embodiment of the method, the IEs are flakes, or at least some of the IEs are flakes. A thickness of the flakes is between 0.05 micrometer and 10 micrometer and a surface area of a face of the flake is between 0.2 square mi ^¬ crometers and 0.2 square millimeters. The flakes provide in- creased surface area and thus further aid catalysis for the electrochemical conversion process.

In another embodiment of the method, at least some of the IEs are fibers and at least some of the IEs are flakes. The at least some of the IEs that are fibers are same in structure and properties as described herein in the embodiment above and the at least some of the IEs that are flakes are same in structure and properties as described herein in the embodi ^¬ ment above .

The IEs may be formed entirely and only of a metal and/or a metal alloy. However, in another embodiment of the method, the IEs have a core material forming a core, coated with a layer of conductive material having the catalytic property for the electrochemical conversion process, for example the core formed of a cost-effective metal e.g. steel, or a poly ^¬ mer e.g. polypropylene, or carbon nanotubes, that is coated or covered with a homogenous outer layer of the conductive material having the catalytic property, or simply put the ac- tive catalytic material for the electrochemical conversion process for example silver, copper, silver alloy, or copper alloy. Alternatively, when the IEs have a core material, forming a core, coated with a layer of conductive material having the catalytic property, as mentioned hereinabove, the layer of conductive material having the catalytic property i.e. the outer layer may additionally have nanoparticles which due to the smaller size will provide a large surface area and thus provide more catalytic active sites. In another embodiment of the method, the organic polymer binding material is an organic polymer such as polysulphone, polyvinylidene fluoride, polyacrylonitrile,

polyethyleneoxide, polymethylmethacrylate, or copolymers thereof. The polymers have hydrophobic property, high heat resistance, oxidation/reduction resistance, and durability and sheet forming properties. In another embodiment of the method, the solvent is one of N- methyl-2-pyrrolidone (NMP) , N-ethyl-2-pyrrolidone (NEP) , N,N- dimethylformamide (DMF) , formamide, dimethylsulphoxide

(DMSO) , N, -dimethylacetamide (DMAC) , acetonitrile and mix ^¬ tures thereof. These provide examples for realizing the meth- od of the present technique by providing a solvent for the solute i.e. the organic polymer binding material.

In another embodiment of the method, in forming the sheet a layer of the casting suspension with a constant thickness is spread on the inert surface. Thus the GDE so formed has con ^¬ stant thickness thereby ensuring uniform properties at dif ^¬ ferent parts of the GDE.

In another embodiment of the method, the solvent extraction is performed by evaporating the solvent from the sheet. In yet another embodiment of the method, the solvent extraction is performed by leaching the solvent out of the sheet wherein the solvent is leached out of the sheet by immersing the sheet into and/or washing the sheet using a non-solvent such as water, an alcohol, and a combination thereof. In the meth ^¬ od, only one of the solvent extraction by evaporation and the solvent extraction by leaching may be performed or alterna ^¬ tively the solvent extraction by evaporation and the solvent extraction by leaching may both be performed one after the other in any order. This provides simple and effective tech ^¬ niques for solvent extraction. In another embodiment of the method, in preparing the casting suspension a metal oxide and/or a metal hydroxide is added to the solution, e.g. Copper oxide, Zirconium oxide as catalyst or Zinc oxide as co-catalyst for the electrochemical conver- sion process.

In another embodiment of the method, in preparing the casting suspension, a pore forming material is added to the solution. Furthermore, the method includes removal of pore forming ma- terial from the sheet simultaneous with and/or subsequent to the extracting the solvent from the sheet. The pore forming material is a polymer pore former and/or a metallic pore for ^¬ mer for example, but not limited to, Zinc oxide,

polyvinylpyrrolidone (PVP) , so on and so forth. Thus the overall porosity of the GDE is modified as required for the use of the GDE.

In another aspect of the present technique, a gas diffusion electrode for use in an electrochemical conversion process is presented. The GDE of the present technique is formed by the method according to the above mentioned aspect of the present technique. The GDE of the present technique does not include as an element a pre-formed conductive solid support as the core or substrate of the GDE. Thus the GDE of the present technique is not limited by the dimensions of the pre-formed conductive solid support, and is physically strong, durable, economical and functional in spite of absence of the pre ^¬ formed conductive solid support. The self-supporting electri ^¬ cally-conductive network formed within the sheet of the GDE of the present technique provides physical integrity, mechan ^¬ ical strength, electrical conductivity, and catalytic proper ^¬ ty to the GDE.

The present technique is further described hereinafter with reference to illustrated embodiments shown in the accompany ^¬ ing drawing, in which: FIG 1 depicts a flow chart showing an exemplary embodi ^¬ ment of a method of the present technique;

FIG 2 depicts a flow chart showing another exemplary embodiment of the method of the present technique;

FIG 3 schematically represents an exemplary embodiment of an interconnecting entity as an un-branched fiber;

FIG 4 schematically represents another exemplary embodi ^¬ ment of the interconnecting entity as a branched fiber;

FIG 5 schematically represents yet another exemplary em ^¬ bodiment of the interconnecting entity as a flake;

FIG 6 schematically represents a cross-section of another exemplary embodiment of the interconnecting entity; and

FIG 7 schematically represents an exemplary embodiment of a sheet and a self-supporting electrically conduc ^¬ tive network formed through volume of the sheet; in accordance with aspects of the present technique.

Hereinafter, above-mentioned and other features of the pre ^¬ sent technique are described in details. Various embodiments are described with reference to the drawing, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of ex ^¬ planation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodi ^¬ ments. It may be noted that the illustrated embodiments are intended to explain, and not to limit the invention. It may be evident that such embodiments may be practiced without these specific details. The basic idea of the present technique is to provide inter ^¬ connecting entities, e.g. fibers, flakes, in forming the gas diffusion electrode wherein the interconnecting entities is formed of a material having catalytic property for the elec- trochemical conversion process. Furthermore, such an amount of the interconnecting entities is used, and owing to generally extended structure of the interconnecting entities, that the interconnecting entities while embedded in the sheet form a self-supporting electrically-conductive network that pro- vides mechanical strength and physical integrity to other components of the sheet that forms the gas diffusion elec ^¬ trode, in short the interconnecting entity holds the sheet together. Additionally the self-supporting electrically- conductive network, formed by physical linking or touching or interconnections between the interconnecting entities, runs through the volume of the gas diffusion electrode and pro ^¬ vides catalytic properties and electrical conductivity to the gas diffusion electrode. In other words the idea of the pre ^¬ sent technique is to use the interconnecting entities, and the self-supporting electrically-conductive network formed as a result of the interconnecting entities, for mechanical re ^¬ inforcement, one or more catalytic properties, structural in ^¬ tegrity and electrical conductivity of the gas diffusion electrode .

FIG 1 depicts a flow chart representing an exemplary embodiment of a method 100 in accordance with the present tech ^¬ nique. FIGs 3 to 7 schematically represent different elements or components used in the method 100 of FIG 1, and are used in combination of FIG 1 for explaining the method 100. The method 100 is for making a gas diffusion electrode, hereinaf ^¬ ter also referred to as the GDE, for use in an electrochemical conversion process, such as for use in an electrochemical conversion of carbon dioxide to carbon monoxide. The gas dif- fusion electrode is particularly suited for fuel cells, and other suitable electrolysers that require a gas-gas or a gas- liquid interaction. In the method 100, in a step 5, interconnecting entities 6 (shown in FIGs 3 to 7) and an amount of the interconnecting entities 6 is selected. The interconnecting entities 6 are selected such that the interconnecting entities 6 include or are formed of one or more materials, for example a metal and/or a metal alloy, that is electrically conducting and a catalyst for the electrochemical reactions that result into the electrochemical conversion for which the GDE is intended to be used. For example, if the GDE being made by the present technique is intended to be used for electrochemical conver ^¬ sion of carbon dioxide to carbon monoxide, then the interconnecting entities 6 selected includes or is formed of silver, silver alloy, copper, copper alloy, and so on and so forth. The silver, silver alloy, etc act as catalysts for one or more of the electrochemical reactions that result in conver ^¬ sion of carbon dioxide to carbon monoxide. Furthermore, sil ^¬ ver, silver alloy, copper, copper alloy, etc are electrically conducting. Furthermore, the interconnecting entities 6 have a physical structure that promotes interlinking when adequate amount of the interconnecting entities 6, hereinafter IEs 6, are confined in a suitable volume, for example the IEs 6 may be elongated rod shaped or fibers - un-branched 7 as shown in FIG 3 or branched 8 as shown in FIG 4, or may be flakes 9 as shown in FIG 5, as opposed to small particles. The fibers 7,8 or the flakes 9 when concentrated or limited in given volume physically touch each other forming more interconnections as compared to rounded or compact particles concentrated or lim ^¬ ited in the given volume. This is explained later in the pre ^¬ sent disclosure in more details.

Each of the FIGs 3, 4 and 5 depict one unit or a single in ^¬ terconnecting entity, and for the present technique a plural ^¬ ity of or many such single interconnecting entities 6 are re ^¬ quired and this represents the amount of the IEs 6. The amount of IEs 6 means numbers of the single IE 6 and can be measured in weight percentage, volume percentage, etc. In some exemplary embodiments, the IEs 6 may have only one type of IE 6, for example each and every IE 6 is the un-branched fiber 7 of FIG 3; or each and every IE 6 is the branched fi ^¬ ber 8 of FIG 4; or each and every IE 6 is the flake 9 of FIG 5. In other exemplary embodiments, the IEs 6 may have more than one type of IE 6, for example some of the IEs 6 are the un-branched fiber 7 of FIG 3 and some of the other IEs 6 are the branched fiber 8 of FIG 4; or some of the IEs 6 are the un-branched fiber 7 of FIG 3 and some of the other IEs 6 are the flakes 9 of FIG 5; or some of the IEs 6 are the branched fiber 8 of FIG 4 and some of the other IEs 6 are the flakes 9 of FIG 5; or some of the IEs 6 are the un-branched fiber 7 of FIG 3 and some of the other IEs 6 are the branched fiber 8 of FIG 4 and some other of the other IEs are the flake 9 of FIG 5. A net length of the fibers 7, 8 i.e. simply length of the un- branched fiber 7 of FIG 3 or total length of all the branches added with length of the part of the branched fiber 8 from which the branches arise as shown in FIG 4, is between 0.1 micrometer and 1000 micrometer. The average diameter of the fibers 7,8, i.e. simply diameter of the un-branched fiber 7 of FIG 3 or average of diameters of all the branches and of diameter of the part of the branched fiber 8 from which the branches arise as shown in FIG 4, is between 0.05 micrometer and 50 micrometer. It may be noted that in FIG 3 and 4, the fibers 7,8 are schematically represented and not shown to have a diameter. A thickness of the flakes 9 is between 0.05 micrometer and 10 micrometer and a surface area of a face 92 of the flake 9 is between 0.2 square micrometers and 0.2 square millimeters as shown in FIG 5.

The IEs 6 i.e. the fibers 7,8 and/or flakes 9 may be formed entirely and only of a metal and/or a metal alloy, example silver, silver alloy, copper, copper alloy, etc. However, in another embodiment, the IEs 6 i.e. the fibers 7,8 and/or flakes 9 have a core material forming a core 62, coated with a layer 64 of conductive material having the catalytic prop ^¬ erty for the electrochemical conversion process, for example the core 62 formed of a cost-effective metal or a material with a desired property such as high mechanical strength e.g. steel, or a polymer e.g. polypropylene, or carbon nanotubes, that is coated or covered with a homogenous outer layer 64 of the conductive material having the catalytic property, or simply put the active catalytic material for the electrochem ^¬ ical conversion process for example silver, copper, silver alloy, or copper alloy. Alternatively, when the IEs 6 i.e. the fibers 7,8 and/or flakes 9 have the core 62 coated with the layer 64 of conductive material having the catalytic property, the layer 64 may additionally have nanoparticles (not shown) dispersed in otherwise homogeneous layer 64 of the conductive material. The nanoparticales may be formed of catalytic or co-catalytic material for example a metal or a metal oxide or metal hydroxide such as copper oxide, Zirconi- urn oxide as catalyst or Zinc oxide as co-catalyst.

Referring to FIG 1, in the method 100, in a step 10 a casting suspension is prepared by mixing the IE 6 selected in step 5 into a solution. The solution has an organic polymer binding material as a solute and the solution further has a solvent for the solute.

To make the casting suspension, the solution of the organic polymer binding material, such as a polymer binding agent for example polysulphone is first prepared in a solvent such as N-methyl-2-pyrrolidone (NMP) , for example in a proportion of 10 to 30 percent by weight binding agent in relation to the amount of solvent. Other examples of suitable materials to be used as the organic binging material are, but not limited to, polyvinylidene fluoride (PVDF) , polyacrylonitrile (PAN) , polyethyleneoxide (PEO) , polymethylmethacrylate (PMMA) , or copolymers thereof. Other examples of the solvent include, but not limited to, N-ethyl-2-pyrrolidone (NEP) , N,N- dimethylformamide (DMF) , formamide, dimethylsulphoxide

(DMSO) , N, -dimethylacetamide (DMAC) , acetonitrile and mix ^¬ tures thereof. Preferably to the solution of the organic pol ^¬ ymer binding material in a suitable solvent, the IEs 6 are added and mixed well to evenly distribute the IEs 6 in the solution .

The amount of the IEs is selected in step 5 to mix with the solution to form the casting suspension prepared in step 10, such that the finally formed GDE has a dry ratio of 90/10 wt% IEs 6 / organic binding polymer content, for example, the amount of the IEs 6 in the finished GDE could be between 5 to 95 wt% of the dry weight of the GDE. The preferred range would be between 80 wt% and 95 wt%. An example of the casting suspension would include NMP between 10 and 70 wt%, silver fibers as IEs 6 between 1.5 wt% and 85 wt% preferably 50 wt% and 75 wt%, and organic polymer binding material between 2.5 wt% and 30 wt%.

Subsequently, in the method 100, in a step 20, a sheet 22 as shown in FIG 7 is formed, from the casting suspension so prepared in step 10, by spreading the casting suspension on an inert surface such as a glass surface. By means of a pouring device the casting suspension, on a glass surface or any other inert surface, is spread evenly or is spread first and then evened out with a blade or wipe action to form a sub ^¬ stantially even layer of the casting suspension on the inert surface, for example a layer of 50 to 2,000 micrometer of the casting suspension is applied onto the glass surface. It may be noted that formation of any bubbles in the sheet 22 are avoided. The casting suspension may be allowed to set on the inert surface by letting the casting suspension to rest on the inert surface for several hours .

The sheet 22 formed in step 20 has predetermined dimensions, i.e. a length, a width and a thickness of the sheet 22 is predetermined. The amount of the IEs 6 is selected in step 5 such that the IEs 6 physically contact or interlink or inter- connect with each other to form a self-supporting electrically-conductive network 50, hereinafter also referred to as the network 50, of the IEs 6 when the casting suspension is spread in the predetermined dimensions to form the sheet 22, as shown in FIG 7. The self-supporting electrically- conductive network 50 means the IEs 6 are linked so as to form an electrically conductive path through the network 50. The phrase ^λ self-supporting' as used herein means when IEs 6 are limited in a volume defined by the predetermined dimen ^¬ sions of the sheet 22, then the IEs 6 interlink with one an ^¬ other to form a network, or a web, or a criss-cross, or a grid, or a lattice, or a matrix, or a webbing that runs through the volume of the sheet 22 and supports or reinforces the sheet 22 i.e. the other components or constituents of the sheet 22 such as the organic polymer binding material.

Finally in the method 100, in a step 30 the solvent, for ex ^¬ ample the NMP, is extracted from the sheet 22 so formed on the inert surface and thus forming the gas diffusion elec ^¬ trode of the present technique. Extraction of the solvent from the sheet 22 has been explained in further details here ^¬ inafter with respect to FIG 2. FIG 2, in combination with FIG 1, represents a flow chart of various other exemplary embodiments of the present technique. As shown in FIG 2, in step 30 of the method 100, the extrac ^¬ tion of the solvent from the sheet 22 may be performed either by evaporating the solvent from the sheet 22 in step 32 or by leaching the solvent from the sheet 22 as shown in step 34 or by both i.e. first by evaporating 32 the solvent from the sheet 22 followed by leaching 34 the solvent from the sheet 22 or first by leaching 34 the solvent from the sheet 22 fol ^¬ lowed by evaporating 32 the solvent from the sheet 22. The step of evaporating the solvent is performed by letting the sheet 22, either still on the glass surface or removed from the glass surface, to stand for up to several hours. Subject ^¬ ing the sheet 22 to elevated temperatures increases the rate of evaporating of the solvent from the sheet 22 thereby fa- cilitating the step 32.

In the step 34 of leaching the solvent from the sheet 22, the solvent is leached out of the sheet 22 by immersing the sheet 22 into and/or washing the sheet 22 using a non-solvent such as water, an alcohol, and a combination thereof. The sheet 22 as set on the inert surface or the sheet 22 removed from the inert surface is immersed in the non-solvent, preferably at room temperature. Suitable types of alcohol are ethanol, but especially isopropyl alcohol. Usually, an immersion time of 20 to 40 minutes is sufficient. The major part of the solvent is extracted in the non-solvent. The remaining solvent is re ^¬ moved by immersing the sheet 22 in a water bath for several hours.

Furthermore, as depicted in FIG 2, in another exemplary em ^¬ bodiment of the method 100, in the step 10 of forming the casting suspension, a metal oxide and/or a metal hydroxide, such as such as copper oxide, Zirconium oxide as catalyst or Zinc oxide as co-catalyst is added to the solution in a step 12. Additionally, in step 10 of preparing the casting suspension in a step 14 a pore forming material is added to the so ^¬ lution either along with or subsequent to the addition of the organic polymer binding material and/or the IEs 6. The pore forming material may be, but not limited to, Zinc oxide, a polymer such as polyvinylpyrrolidone (PVP) , crosslinked polyvinylpyrrolidone (PVPP) , poly (vinyl alcohol), poly (vinyl acetate), methyl cellulose and polyethylene oxide.

Preferably, at least one pore forming material is added to the casting suspension which advances the pore formation. When using PVP as the pore forming material a suitable amount lies between 0.5 percent by weight and 2 percent by weight, for example 0.7 percent by weight (%wt) of the entire compo ^¬ sition of the casting suspension. However, if porosity of the GDE is desired to be high, a suitable amount of the pore forming material may be between 0.5 %wt and 50 %wt . Preferably, the pore-forming material is added to the suspen ^¬ sion after the organic polymer binding material has been dissolved. Alternatively, first the pore-forming material is dissolved in the solvent, after which the organic polymer binding material is added to the formed solution, preferably at an increased temperature, for example at 70 to 75 degrees Celsius . Furthermore, the method 100 includes a step 40 of removing the pore forming material from the sheet 22 simultaneous with the step 30 of extracting the solvent from the sheet 22, as depicted in FIG 2. In another embodiment of the method 100, as depicted in FIG 1, the step 40 of removing the pore form- ing material from the sheet 22 is performed subsequent to the step 30 of extracting the solvent from the sheet 22. It may be noted that, in another embodiment of the method 100, the step 40 of removing the pore forming material from the sheet is simultaneous with and continues subsequent to the extract- ing the solvent from the sheet 22. The step 40 is performed depending on the pore forming material that was used in the step 14, for example when the pore forming material is Zinc oxide the step 40 is performed by subjecting the sheet 22 to acidic or alkaline bath, whereas when the pore forming mate- rial is PVP the step 40 is performed by subjecting the sheet 22 to boiling water bath. The removal of the pore-forming material provides pores on a surface and internally in the gas diffusion electrode. In another aspect of the present technique, a gas diffusion electrode for an electrochemical conversion processes is pre ^¬ sented. The gas diffusion electrode of the present technique is formed by the method 100 according to the above mentioned aspect of the present technique as described in reference to FIGs 1 and 2, in combination with FIGs 3 to 7. It may be not ^¬ ed that the gas diffusion electrode of the present technique does not include a pre-formed solid support as the core or substrate of the gas diffusion electrode. While the present technique has been described in detail with reference to certain embodiments, it should be appreciated that the present technique is not limited to those precise embodiments. Rather, in view of the present disclosure which describes exemplary modes for practicing the invention, many modifications and variations would present themselves, to those skilled in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.